1 (define (union-set set1 set2)
2   (cond ((null? set1) set2)
3 	((null? set2) set1)
4 	((element-of-set? (car set1) set2) (union-set (cdr set1) set2))
5 	(else (cons (car set1) (union-set (cdr set1) set2)))))